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The RockAuto pistons are Sealed Power cast
pistons. They cost almost twice as much as 440 cast pistons, about $150
more than standard weight 440 forged pistons, and almost as much as lightweight
forged 440 pistons.
The 9.7 CR rating means nothing. The
compression height for these pistons is an even 2.0 inches. Plug that into
a calculator and the deck clearance calculates to be -.082.
See
This is for a 3.75 stroke, 6.786 rod, and 10.725
crank centerline to deck (common dimensions for 413's and 440's, only the bore
size is different).
With that deck clearance and small chamber head
with FelPro gaskets, you end up with an actual CR of 8.98. That's assuming
the heads are 78 CC, which is about what the early small chamber heads are
supposed to be. If they are actually 80 cc's, the CR drops to 8.84.
If you put a set of later large chamber heads (i.e., 906, 452, 346 etc) because
they flow better or to get hardened seats, the CR drops to 8.3, and that's
assuming the large chamber heads are only 88 cc, which is probably on the low
side. With 90 cc. heads, the CR drops to 8.17.
You can play with these numbers on this compression
ratio calculator:
Remember that any head gaskets you buy for a 413
are going to have large enough opening to fit a .060 over 440, which means the
gasket opening is going to be at least 4.38, and is probably a bit
larger.
The FelPro blue head gaskets are .039 thick, while
the replacement type steel shim are about .021. The steel shim gaskets
will bump the CR up about three tenths, but don't seal as well if the surface
isn't absolutely true and generally aren't used on a rebuild, although I have
them on my 413 just to get all the CR help I could.
The only way to really know what the actual CR
is, is to CC the heads and check the actual deck clearance, which can vary from
one 413 to another or even one cylinder to another due to slight variations in
rod length and deck height. (Typically, the deck is not
machined true to the crank centerline, and can be taller on the intake
side than the exhaust, or on the front than the rear, and vice
versa).
All boils down to this: the only readily available,
off the shelf pistons for a 413 are going to be .080 from the deck at TDC.
Want better than that and you are going to have to have them custom
made.
Curtis
59 Coronet
56 Belvedere
----- Original Message -----
Sent: Thursday, October 23, 2003 8:34
AM
Subject: [FWDLK] 413 pistons
The 300 listserver had a bit of mail recently
regarding 413 pistons. Here's what I found.
The Rockauto site is a catalog you have to
download, but it had the cheapest pistons at about $68 each if I remember
rightly. 9.7:1 cr they say.
What happened to Badger? Have they departed
the planet? I have a new set of .030 413's cast Badger pistons,
10:1 cr I bought about 1995 for $300. Moly rings were another $70,
they were Hastings rings and I bought them from Hastings
themselves.
Dave........
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